FIG. 16 shows a known electric system for an electric vehicle, which uses a battery as a power supply. The electric system of FIG. 16 includes a main battery 1 as a dc power supply, for which a chemical battery is typically used as a high energy type battery, an ac motor 2 for driving the vehicle, a variable-voltage, variable-frequency, voltage type inverter 3 for driving the motor, reduction gears 4, a differential gear system 5, and wheels 6. The electric system also includes auxiliary electric motors 7 (only one of which is shown in FIG. 16) for driving accessories, such as an air conditioner, a power steering system, and various pumps, and an inverter 8 for driving such an auxiliary electric motor 7. The electric system further includes a capacitor 9 as a high power type battery, for which an electric double layer capacitor as a physical capacitor is used, and a chopper 10 inserted between the capacitor 9 and the main battery 1.
Generally, the high power type battery discharges electric power during acceleration of the vehicle, and absorbs braking energy of the vehicle during deceleration. Accordingly, the voltage of the capacitor 9 is lowered due to discharge of the power while the vehicle is being accelerated. While a brake is being applied to the vehicle, on the other hand, the voltage of the capacitor 9 is increased due to absorption of the power into the capacitor 9. The chopper 10 is inserted between the capacitor 9 whose voltage varies to a large extent, and the main battery 1 whose voltage is almost constant, so that power can be transferred between the capacitor 9 and the battery 1.
FIG. 17 shows an electric system of a series type hybrid electric vehicle, in which the same reference numerals as used in FIG. 16 are used for identifying the same components. The system of FIG. 17 includes an engine 11, a generator 12, a converter 13, and a battery 14 provided for absorbing a difference between the power generated by the generator 12 and the power consumed by the electric motor 2. In the series type system, electric power for running the vehicle is generated by the engine 11, generator 12 and the converter 13, and the power thus generated is used for driving the motor 2 via the inverter 3. Although various types of series type systems are available, the system shown in FIG. 17 uses a chemical battery 14 as a battery for absorbing excess power. Since the battery 14 is not of a high power type, the capacitor 9, as a high power type battery, is connected in parallel with the battery 14 with the chopper 10 inserted therebetween.
Generally, the output of the generator is not the maximum power required for acceleration or deceleration of the vehicle, but determined by the power required for constant-speed running. Thus, the power generated by the generator is smaller than the maximum power. During acceleration or deceleration of the vehicle that requires high power, therefore, electric power is supplied from or received by the capacitor 9, via the chopper 10. The chemical battery 14 absorbs power corresponding to a difference between the power generated by the generator 12, and the power consumed by the motor 2 and accessories (not illustrated). In the case where the engine 11 is stopped, and the vehicle runs only with the chemical battery 14, the battery 14 is required to provide a sufficiently high energy.
FIG. 18 shows an electric system of a parallel type hybrid electric vehicle, wherein the same reference numerals as used in FIGS. 16 and 17 are used for identifying the same components. The system of FIG. 18 includes an engine 15 for the parallel system, a generator 16, a transmission 17, a chemical battery 18, a converter (inverter) 19, an electric double layer capacitor 20 as a high power type battery, and a chopper 21. The parallel type hybrid electric vehicle may travel only due to the power of the engine 15 transmitted to the wheels through the transmission 17, or due to the electric power of the chemical battery 18 transmitted through the converter (inverter) 19 and generator 16, or due to the power of both of the engine 15 and battery 18, and each power transmitting system is selectively used depending upon applications. The parallel type vehicle also requires a high power type battery for absorbing regenerative power during braking of the vehicle, and therefore the electric double layer capacitor 20 is connected in parallel with the battery 18 via the chopper 21.
As shown in the known examples as described above, both high energy type battery and high power type battery are needed as a power supply for an electric vehicle. The high energy type battery determines the running distance of the vehicle per charge, and the high power type battery determines the acceleration performance and regenerative braking performance. If the braking energy can be sufficiently regenerated into the battery upon each braking of the vehicle, the electric vehicle provides a considerably high energy-saving effect.
The operations for driving electric vehicles are the same as those for driving current gasoline-powered vehicles, and the number of braking operations reaches as much as several tens of thousands of times. To achieve a further improved energy-saving effect, the electric vehicle is required to endure accelerating and decelerating operations performed as much as several tens of thousands of times. Namely, the battery used in the electric vehicle is desired to withstand several tens of thousands of operations for charging and discharging large power. It is, however, difficult for the currently used battery, which is typically a chemical battery, to perform several tens of thousands of operations for charging and discharging large power, and such a battery can perform at most several thousands of charging and discharging operations. In the current electric vehicles, therefore, it is necessary to replace the chemical battery with a new one after every suitable period of time, or use a high power type battery, such as an electric double layer capacitor, in addition to the chemical battery.
FIG. 19 shows (a) vehicle speed V, (b) inverter input Pi, (c) voltage Vc of the electric double layer capacitor, and input voltage Vi of the inverter, and (d) output Ps of the dc power supply (chemical battery), corresponding to each of the operating modes (accelerating mode, constant-speed running mode, and decelerating mode) of the electric vehicle.
FIG. 20 shows one example of chopper used in the systems of FIG. 16 to FIG. 18, more specifically, illustrates the chopper 10 of FIG. 16 by way of example. The chopper 10 includes switch portions 10b, 10c, in which transistors 10b1, 10c1 are respectively connected in parallel with diodes 10b2, 10c2 in the reverse direction. Namely, the negative side of each of the transistors 10b1, 10c1 is connected to the positive side of a corresponding one of the diodes 10b2, 10c3. The chopper 10 further includes a current smoothing reactor 10a, and a voltage smoothing capacitor 10d.
Electric vehicles, which are used for the same purpose as conventional gasoline-powered vehicles, are desired to satisfy the same demands as made in the gasoline-powered vehicles, for example, a long running distance per charge, high acceleration and deceleration performance, high fuel efficiency, small size and weight of its components, and low cost. Of these demands, the running performance and the fuel efficiency have been improved by using the high power type battery, such as an electric double layer capacitor, or utilizing two or more kinds of power sources to provide a hybrid system.
Even with the improved performance of the electric vehicle, there is still a significant problem in the high power type battery portion, in particular, chopper portion as shown in FIG. 20. Since the chopper portion must be of high power type, and have almost the same capacity as the inverter for driving the motor, it has been desired to reduce the size and weight of the chopper, and its cost.